The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a lithium-containing composite oxide used as a positive electrode active material for a non-aqueous electrolyte secondary battery.
Since lithium-containing composite oxides can intercalate and deintercalate lithium ions reversibly and repeatedly, attempts have been made to use them as a positive electrode material for so-called lithium ion secondary batteries. Among lithium-containing composite oxides, particularly promising are Li—Mn composite oxide, Li—Co composite oxide and Li—Ni composite oxide, some of which have already been commercialized. In order to improve the characteristics as the positive electrode material, attempts have also been made to partially replace a transition metal element of a conventional lithium-containing composite oxide with other metal element. Those currently proposed are Li—Mn—Ni composite oxide, Li—Co—Al composite oxide and Li—Mg—Co composite oxide.
In the development and improvement process of electrode materials for obtaining excellent battery characteristics, the elements for constituting the electrode material and the composition thereof are first selected. Secondly, the optimal baking conditions for raw materials for obtaining the electrode material are determined. Under this process, because crystallographic or electrochemical basic data are examined, an evaluation and analysis using a small amount of sample often provides sufficient results.
Subsequently, an examination is made to check if the improved electrode material is applicable to the actual production of electrode plates. Under this process, firstly, the electrode material is dispersed in a liquid component to give a paste, and the quality of the paste is checked. Secondly, the paste is applied onto a current collector, followed by drying to give an electrode plate, and the electrode plate is tested for flexibility, mechanical strength, etc.
The quality of the paste is determined from an industrial point of view. More specifically, because the paste is sometimes sit for a while before it is applied onto a current collector, the stability against change with time is examined. Industrially speaking, for example, a paste as a non-Newtonian fluid is desired because the amount of the electrode material that settles out within the paste with time is small, and it is easy to apply. Desirably, the viscosity of the paste does not vary greatly with time.
Conventionally, the improvement of lithium-containing composite oxides has been done focusing on the powder properties thereof. For example, Japanese Laid-Open Patent Publications Nos. Hei 10-125325 and Hei 10-312792 focus on typical powder properties such as average particle size and specific surface area.
Even a lithium-containing composite oxide expected to be suitable for the production of electrode plates based on the findings of the past and the known powder properties can cause a problem when it is practically used in the production of electrode plates. That is, when an electrode material mixture paste prepared by mixing a lithium-containing composite oxide with a conductive material, a binder and a liquid component is applied onto a current collector, followed by drying to give an electrode plate, the material mixture might be separated and detached from the current collector.